Globally anisotropic high porosity silica aerogels

We discuss two methods by which high porosity silica aerogels can be engineered to exhibit global anisotropy. First, anisotropy can be introduced with axial strain (i.e. axial compression). In addition, intrinsic anisotropy can result during growth and drying stages and, suitably controlled, it can be correlated with preferential radial shrinkage in cylindrical samples. We have performed small angle X-ray scattering (SAXS) to characterize these two types of anisotropy. We show that global anisotropy originating from either strain or shrinkage leads to optical birefringence and that optical cross-polarization studies are a useful characterization of the uniformity of the imposed global anisotropy.     

Moment Lyapunov exponents of a two-dimensional system under bounded noise parametric excitation

The moment Lyapunov exponents of a two-dimensional system under bounded noise parametric excitation are studied in this paper. The method of regular perturbation is applied to obtain weak noise expansions of the moment Lyapunov exponent, Lyapunov exponent, and stability index in terms of the small fluctuation parameter.     

Fluid-fluid phase separation in colloid-polymer mixtures studied with small angle light scattering and light microscopy

Mixtures of colloidal silica spheres and polydimethylsiloxane in cyclohexane with a colloid-polymer size ratio of about one were found to phase separate into two fluid phases, one which is colloid-rich and one which is colloid-poor. In this work the phase separation kinetics of this fluid-fluid phase separation is studied for different compositions of the colloid-polymer mixtures, and at several degrees of supersaturation, with small angle light scattering and with light microscopy. The small angle light scattering curve exhibits a peak that grows in intensity and that shifts to smaller wave vector with time. The characteristic length scale that is obtained from the scattering peak is of the order of a few μm, in agreement with observations by light microscopy. The domain size increases with time as , which might be an indication of coarsening by diffusion and coalescence, like in the case of binary liquid mixtures and polymer blends. For sufficiently low degrees of supersaturation the angular scattering intensity curves satisfy dynamical scaling behavior.     

Induced smectic C* phases

Induced ferroelectric S*_C phases are formed by non-chiral S_C host phases doped with chiral dipolar guest molecules. In those mixtures the spontaneous polarization P_s and the tilt angle Θ has been investigated as a function of the mole fraction x_G of the chiral dopant. In most cases the reduced polarization P₀ = P_S/ sin Θ has been found to depend linearly on x_G. The polarization power which is defined by δ_P=(∂P₀/∂x_G)ΔT is discussed in terms of the molecular structure of the chiral dopants. There are systems in which P₀(x_G) deviates positively from linearity. This behaviour can be understood by considering a local field correction to P₀. By assuming a local field of Lorentz type a theoretical relation for P₀(x_G) has been derived which explains the experimental results. The effect of a local field is considerable if the transverse dipole moment and the polarizability of the chiral dopant are large.     

A novel derivatization method for peptides to increase sensitivity and backbone fragmentation in liquid secondary-ion mass spectra.

Derivatization is used to increase both negative-ion sensitivity and positive-ion sequence information in the liquid secondary-ion mass spectra (LSIMS) of a series of peptides. The derivatization method involves acylation with pentafluorobenzoyl fluoride in a single-step reaction, and the reaction mixture is applied directly to the probe tip for analysis. Acylation takes place at the unprotected N-terminus, tyrosine, and lysine. The derivatives exhibit increased signal-to-noise ratio for [M-H]- ions, especially where there is not already an acidic amino acid residue in the peptide. In positive-ion LSIMS, the N-terminal group acts to retain the charge at the N-terminus, simplifying the fragmentation by producing N-terminal fragment ions. It also increases positive-ion fragmentation, sometimes very dramatically, making sequence determination more straightforward. The simplicity of the process, together with the enhancements it provides, make this a generally useful method for obtaining peptide structural information.     

1.2 Anorganische analyse

Ohne Zusammenfassung